The way aspects work and all related facets of the fate point economy are core. I see this changing very minimally over time, apart from some optional ways of handling certain things, like having different "flavors" of fate points as per Fred's Dictionary of Mu conversion.

The current system of determining action success and permutations via the assignment of shifts is core and replaces older iterations of MoS and MoF entirely.

Phase-based or "on the fly" character creation is core, though no certain number of phases or assigned aspects is considered default in any sense, nor is the specific content of those phases. This means that ideas like the "guest star" phases from Spirit of the Century (SotC) are not necessarily default concepts.

Skills and trappings as per SotC are core, though the pyramid may not considered a default arrangement. Fate v2 also had a column structure, and had the notion of point-buy that allowed some flexibility to the shape of the character's skill set, which SotC does not have. And Fred's recent Dictionary of Mu conversion ditched the structure entirely in favor of point-spend limits in different categories. So, all that's way up in the air.

Most importantly, the use of stunts is not considered a core construct at this time - SotC used them for specific reasons, and Dresden will too, for other specific reasons. This includes all sub-systems introduced under stunts, like the companion rules and whatnot.

Most of the other subsystems that do not deal with core resolution are not core, notably the chase mechanics and how SotC deals with vehicles and gadgets, the Might/weight chart, the Resources/cost chart, the explosions rules from SotC, etc.

The conflict procedure from SotC is not necessarily a core construct - if you recall, Fate v2 had three methods of conflict resolution.

Any of the ratio associations from SotC (your refresh = how many aspects you have, stunts = half your aspects) are not default.

In a theoretical Fate v3 Core document, all the stuff I'm telling you is not core or default would still be included, but with notes about what genres they fit, what game functions they perform, what play styles they fit, or simply as an example of how to construct something specific from the universal concepts. Examples, examples, examples, stretching out to the end of the universe.

So, really, the previous statements made about SotC being about 80% core and 20% optional material are off by a lot; it actually works out more to be like 40% core and 60% optional material. The Dresden Files RPG will very likely be the same, perhaps even more toward the 30/70 end. And I think you can see how this lends itself much better to the modularity that was a design hallmark of Fate v2.